The present invention relates to manufacture of semiconductor integrated circuits and more particularly to a method of chemical mechanical polishing of conductive layers using smart endpoint detection.
Conventional semiconductor devices generally include a semiconductor substrate, usually a silicon substrate, and a plurality of sequentially formed dielectric interlayers such as silicon dioxide and conductive paths or interconnects made of conductive materials. Copper and copper alloys have recently received considerable attention as interconnect materials because of their superior electromigration and low resistivity characteristics. Interconnects are usually formed by filling copper in features or cavities etched into the dielectric interlayers by a metallization process. The preferred method of copper metallization process is electroplating. In an integrated circuit, multiple levels of interconnect networks laterally extend with respect to the substrate surface. Interconnects formed in sequential layers can be electrically connected using vias or contacts. In a typical process, first an insulating layer is formed on the semiconductor substrate. Patterning and etching processes are performed to form features such as trenches and vias in the insulating layer. After coating features on the surface with a barrier and then a seed layer, copper is electroplated to fill the features. However, the plating process, in addition to the filling the features, also results in a copper layer on the top surface of the substrate. This excess copper is called overburden and it should be removed before the subsequent process steps.
FIG. 1A shows an exemplary portion 8 of such plated substrate 9, for example a silicon wafer. It should be noted that the substrate 9 may include devices or other metallic and semiconductor sections, which are not shown in FIG. 1A for the purpose of clarification. As shown in FIG. 1A, features such as a vial 0, and a trench 12 are formed in an insulation layer 14, such as a silicon dioxide layer, that is formed on the substrate 9. The via and the trench 12 as well as top surface 15 of the insulation layer 14 are covered and filled with a deposited copper layer 16 through an electroplating process. Conventionally, after patterning and etching, the insulation layer 14 is first coated with a barrier layer 18, typically, a Ta or Ta/TaN composite layer. The barrier layer 18 coats the via and the trench as well as the surface 15 of the insulation layer to ensure good adhesion and acts as a barrier material to prevent diffusion of the copper into the semiconductor devices and into the insulation layer. Next a seed layer (not shown), which is often a copper layer, is deposited on the barrier layer. The seed layer forms a conductive material base for copper film growth during the subsequent copper deposition. As the copper film is electroplated, the deposited copper layer 16 quickly fills the via 10 but coats the wide trench 12 and the top surface 15 in a conformal manner. When the deposition process is continued to ensure that the trench is also filled, a copper layer or overburden is formed on the substrate 9. Conventionally, after the copper plating, various material removal processes, for example, chemical mechanical polishing (CMP), etching or electroetching, can be used to remove the unwanted overburden layer.
The CMP process conventionally involves pressing a semiconductor wafer or other such substrate against a moving polishing surface that is wetted with a polishing slurry. The slurries may be basic, neutral or acidic and generally contain alumina, ceria, silica or other hard abrasive ceramic particles. The polishing surface is typically a planar pad made of polymeric materials well known in the art of CMP. Some polishing pads contain abrasive particles (fixed abrasive pads). These pads may be used in conjunction with CMP solutions that may not contain any abrasive particles. The polishing slurry or solution may be delivered to the surface of the pad or may be flowed through the pad to its surface if the pad is porous. During a CMP process a wafer carrier holds a wafer to be processed and places the wafer surface on a CMP pad and presses the wafer against the pad with controlled pressure while the pad is rotated. The pad may also be configured as a linear polishing belt that can be moved laterally as a linear belt. The process is performed by moving the wafer against the pad, moving the pad against the wafer or both as polishing slurry is supplied to the interface between the pad and the wafer surface.
As shown in FIG. 1B, CMP is first applied to reduce the thickness of the copper layer down to the barrier layer 18 that covers the top surface 15 of the insulation layer 14. Subsequently, the barrier layer 18 on the top surface is removed to confine the copper and the remaining barrier in the vias 10, 12 and trenches 13. However, during these processes, determining the polishing endpoint, whether the copper layer is polished down to the barrier layer or the barrier layer is polished down to the insulation layer, is one of the important problems in the industry.
U.S. Pat. No. 5,605,760 describes a polishing pad that is made of solid uniform polymer sheet. The polymer sheet is transparent to light at a specified wavelength range. The surface of the polymer sheet does not contain any abrasive material and does not have any intrinsic ability to absorb or transport slurry particles.
More recently, endpoint detection systems have been implemented with rotating pad or linear belt systems having a window or windows in them. In such cases as the pad or the belt moves, it passes over an in-situ monitor that takes reflectance measurements from the wafer surface. Changes in the reflection indicate the endpoint of the polishing process. However, windows opened in the polishing pad can complicate the polishing process and may disturb the homogeneity of the pad or the belt. Additionally, such windows may cause accumulation of polishing byproducts and slurry.
Therefore, a continuing need exists for a method and apparatus which accurately and effectively detects an endpoint on a substrate when the substrate is polished using the CMP processes.
As shown in FIG. 1B, CMP is first applied to reduce the thickness of the copper layer down to the barrier layer 18 that covers the top surface 15 of the insulation layer 14. Subsequently, the barrier layer 18 on the top surface is removed to confine the copper and the remaining barrier in the via 10 and trench 12. However, during these processes, uniform reduction of the thickness of the polished copper layer is one of the important problems in the industry. The thickness uniformity of the metal layer must be maintained while it is processed so that the overpolish after copper endpoint is minimized and the substrate is not over-polished, since overpolishing may cause excessive dishing, erosion and other defects. Further, underpolishing of the copper layer and barrier layers may cause electrical shorts or other defects. The non-uniformity during the polishing process may be due to either a non-uniform polishing process or a non-uniform thickness of the metal layers on the substrate or both.
A uniform polishing process will significantly reduce CMP cost while increasing process throughput. As the wafer sizes become larger, e.g., 300 mm and beyond, a planar reduction of thickness in a uniform manner becomes more difficult due to the larger surface area of the wafer.
Consequently, there is need for an improved method and apparatus for monitoring and maintaining the uniformity of the polished layer when the substrate is polished using CMP processes.
The present invention advantageously provides an in-situ method and apparatus for performing endpoint detection for material removal processes such as CMP.
A second embodiment includes a system that provides an advanced chemical mechanical polishing (CMP) system with smart endpoint detection.
A chemical mechanical polishing (CMP) apparatus for polishing a surface of a workpiece and for detecting a CMP endpoint is presented according to an aspect of the present invention. The CMP apparatus includes an optically transparent polishing belt, a workpiece holder, a support plate, and an optical detection system. The polishing belt, preferably including abrasive particles, polishes the surface of the workpiece and is movable in one or more linear directions. The workpiece holder supports the workpiece and is configured to press the workpiece against the polishing belt. The support plate is adapted to support the polishing belt as the workpiece is pressed against the polishing belt. The optical detection system detects the CMP endpoint and is disposed below the polishing belt. The optical detection system includes a light source and a detector. The light source sends outgoing signals through the support plate and the polishing belt to the surface of the workpiece. The detector receives incoming reflected signals from the surface of the workpiece through the polishing belt and the support plate.
A method of polishing a surface of a workpiece and of detecting a chemical mechanical polishing (CMP) endpoint is presented according to another aspect of the present invention. According to the method, the workpiece is pressed against an optically transparent polishing belt. The polishing belt is supported by a support plate. The surface of the workplace is polished with the polishing belt. The polishing belt is movable in one or more linear directions. Outgoing optical signals are sent from a light source through the support plate and the polishing belt to the surface of the workpiece. The light source is disposed below the polishing belt so that the polishing belt is between the light source and the surface of the workpiece. Incoming reflected optical signals are received from the surface of the workpiece through the polishing belt and the support plate at a detector. The detector is disposed below the polishing belt.
A method of polishing one or more workpieces and of providing chemical mechanical polishing (CMP) endpoint detection is presented according to a further aspect of the present invention. According to the method, an optically transparent polishing belt is provided between a supply area and a receive area. The polishing belt has a first end and a second end and a polishing side and a backside. The first end initially comes off the supply area and is connected to the receive area and the second end remains connected to the receive area. A first workpiece is polished by moving a portion of the polishing belt in one or more linear directions within a polishing area. A first CMP endpoint of the first workpiece is detected using an optical detection system. The optical detection system sends outgoing signals to and receives incoming reflected signals from the first workpiece through the polishing belt. The polishing belt is located between the optical detection system and the first workpiece.
A CMP apparatus for polishing a surface of a workpiece and for detecting a CMP endpoint is presented according to another aspect of the present invention. The CMP apparatus includes a supply spool and a receiving spool, an optically transparent polishing belt, a processing area, a means for moving a section of the polishing belt in one or more linear directions, and a means for detecting a CMP endpoint. The polishing belt has two ends. One end is attached to the supply spool and the other end is attached to the receiving spool. The processing area has a section of the polishing belt in between the two ends. The means for detecting the CMP endpoint sends optical signals to, and receives reflected optical signals from, the surface of the workpiece through the polishing belt. The polishing belt is located between the means for detecting and the workpiece.
A method of polishing a surface of a workpiece and of detecting a CMP endpoint is presented according to a further aspect of the present invention. According to the method, the workpiece is supported such that the surface of the workpiece is exposed to a section of an optically transparent polishing belt in a processing area. The surface of the wafer is polished by moving the section of the polishing belt bidirectional linearly. A CMP endpoint is determined for the workpiece by sending outgoing optical signals through the polishing belt to the workplace and continuously examining the relative intensity of incoming optical signals reflected from the workpiece and received through the polishing belt. The foregoing discussion of aspects of the invention has been provided only by way of introduction. Nothing in this section should be taken as a limitation on the following claims, which define the scope of the invention.
A second exemplary embodiment of the invention includes a polishing station having a workpiece holder, and a flexible polishing pad (e.g. polishing belt). The polishing pad is held against the workpiece by a platen that supplies a fluid against the backside of the pad. The platen includes a number of holes for supplying the fluid and also includes a number of sensors that can detect the endpoint of the workpiece processing. The holes are grouped together to create pressure zones and typically one sensor is associated with each zone, but there may be more or less. A computer receives the sensor signals and controls the fluid flow to optimize the polishing. If, for example, a certain location on the workpiece reaches the endpoint, the computer reduces the fluid flow to that location while maintaining the fluid flow to other areas.
In one aspect of the invention, the fluid controller independently controls the fluid flow to the pressure zones. One feature of this aspect is that the invention can also selectively exhaust fluid from certain holes in the platen to reduce, and even negatively influence, the pressure zones.
In another aspect of the invention, the workpiece is rotated during processing and the platen holes are located concentrically and each concentric ring represents a pressure zone.
In another aspect of the invention, the fluid controller independently controls the fluid flow to the concentric rings on the platen.
In another aspect of the invention, the belt is optically transparent.
In another aspect of the invention, the belt includes windows.
In another aspect of the invention, the sensors are light sensors.
In another aspect of the invention, the sensors are acoustic thickness sensors.
In another aspect of the invention, the sensors use fiber optic threads.
In another aspect of the invention, the workpiece is kept substantially stationary, but may be rotationally and translationally moved during the polishing process. In a preferred aspect of the invention, the translational movement is smaller than a pressure zone area.
Advantages of the invention include the ability to optimally polish the workpiece, thereby saving time and money.